There have been great strides made in the study of laser in recent years. In particular, solid lasers and semiconductor lasers that can emit light of wavelengths ranging from the near infrared region to the infrared region are available in the form of high-powered, small-sized laser devices. Such laser devices are remarkably useful as the light sources for light exposure when printing plates are made by direct transfer of digital data from computers or the like to image forming materials.
Conventionally, a light-sensitive image forming material of a positive-working mode designed for direct plate making, which is used with infrared laser comprises a novolak resin as an alkali-soluble resin. For example, Japanese Patent Application Publication (hereunder referred to as “JP KOKAI”) No. Hei 7-285275 discloses a light-sensitive image forming material of a positive-working mode. The above-mentioned image forming material comprises an alkali-soluble resin having a phenolic hydroxyl group, such as a novolak resin, to which are added light-sensitive compounds of positive-working mode, i.e., a material capable of generating heat after absorbing light, a variety of onium salts, and quinone diazide compounds. At a non-light exposed portion (which will serve as an image portion) in the image forming material, the above-mentioned light-sensitive compounds of positive-working mode serve as a dissolution blocking agent, in other words, work to substantially decrease the solubility of the alkali-soluble resin. At a light exposed portion (which will become a non-image portion), on the other hand, the light-sensitive compounds do not exhibit any dissolution blocking performance by the generation of heat, so that the image forming material is allowed to dissolve in a developing solution, to complete the removal of the image forming material at the non-image portion. Thus, image formation is achieved.
Further, there is proposed a light-sensitive image forming material of positive-working mode comprising a substance capable of absorbing light to generate heat and a resin of which the solubility in alkaline aqueous solutions is changeable depending upon the application of heat as disclosed, for example, in International Publication No. WO 97/39894 and EP-A-0823327. This type of image forming material can achieve image formation in such a manner that one portion of the image forming material corresponding to an image portion exhibits a low solubility in an alkaline aqueous solution, while the other portion which will be a non-image portion shows an increased solubility in the alkaline aqueous solution by the generation of heat, thereby readily allowing the portion corresponding to the non-image portion to be removed by development.
The conventional PS plate for lithographic printing preferably employs a novolak resin for the reasons that the strong interaction between the novolak resin and the dissolution blocking agent makes a big difference in the solubility in the employed developing solution between the light-exposed portion and the non-light exposed portion, and that the ink receptivity is excellent. For the same reasons as mentioned above, it is preferable to use the novolak resin in the light-sensitive image forming material of a positive-working mode used with infrared laser. Thus, a variety of techniques for making a printing plate from the image forming material containing the novolak resin have been studied. For example, it is proposed to add an anionic or amphoteric surfactant serving as a hydrotropic agent to the alkaline developing solution as disclosed in EP-A-908785. Also, JP KOKAI No. 2002-72501 discloses the use of a particular amphoteric surfactant in the alkaline developing solution.
However, there occurs a problem associated with stability when a positive-type PS plate for lithographic printing is made from a light-sensitive image forming material containing a novolak resin. More specifically, to produce such a PS plate, the image forming material in a melted state is coated on an appropriate substrate, dried and set to form a light-sensitive layer on the substrate. After completion of the production of the PS plate for lithographic printing, the sensitivity of the light-sensitive layer deteriorates with the passage of time. Namely, the stability cannot be maintained for an extended period of time.
Research has been conducted on the development of the image forming material that can satisfy both the sensitivity and the long-term stability, and at the same time that can readily be manufactured. In consideration of those factors, a light-sensitive image forming material comprising a novolak resin is proposed, for example, in JP KOKAI No. 2001-350261, the novolak resin comprising xylenol as a constituent monomer. The above-mentioned novolak resin containing as a monomer component xylenol shows a lower solubility in alkaline aqueous solutions as compared with the conventional novolak resins. Therefore, when this type of novolak resin is used for the image forming material, the image forming material constituting a non-image portion once dissolved in the developing solution tends to form an insoluble matter in the developing solution. Such insoluble matter is accumulated and aggregated to form developing sludge in the developing solution after repetition of the development steps, which becomes a factor that will make the development step unstable. To be more specific about the disadvantages caused by the above-mentioned phenomenon, the developing sludge can be deposited on the resultant plate to impair the images formed thereon, and the developing sludge which settles down and precipitates in a processing bath for development makes the maintenance of the processing bath more burdensome. In the case where the PS plate is subjected to a burning treatment, the developing sludge, if still remaining on the non-image portion of the PS plate, will be carbonized, thereby resulting in scumming, in other words, staining the printed matter.
There has thus been an increasing demand for sharp and clear image formation on the printing plate, in particular, higher sharpness and improved reproducibility of elaborate images including a dot portion and a fine line portion, without impairing the formed images by avoiding the disadvantages caused by the insoluble matter in the developing solution resulting from some component constituting the image recording layer of the image forming material.